The nuclear hormone receptor superfamily is composed of ligand-regulated transcription factors and orphan receptors, for which ligands are not identified or may not exist. These receptors perform extremely diverse functions in development, reproduction, and homeostasis by regulating cell growth, differentiation, and apoptosis. Aberrant activities of some hormone receptors have been causally linked to neoplasia. Recently, studies from our group and others on the orphan nuclear receptors ROR-gamma (Retinoic acid receptor-related Orphan Receptor) and its thymus specific isoform ROR-gamma-t demonstrated that this nuclear receptor regulates the survival and cell cycle status of CD4+CD8+ double-positive thymocyte. However, the role of ROR-gamma-t/ROR-gamma in thymocyte differentiation remains largely unknown. In this proposal, we will perform experiments to determine how ROR-gamma-t/ROR-gamma regulates thymocyte positive and negative selection through the use of animals lacking ROR-gamma-t/ROR-gamma expression. Furthermore, we will examine the regulation of pre-TCRa by ROR-gamma-t in vivo. These experiments will provide important new information regarding the role of ROR-gamma-t and ROR-gamma in thymopoiesis and the molecular basis of thymocyte differentiation. In addition, results from this application will also provide insights for the role of ROR-gamma-t in tumorigenesis.